The invention relates to a structural model of the human craniomandibular system.
It has been customary to interpret the complicated connection between the skeletal and muscular components of the craniomandibular system with the help of a diagram which was first proposed by Brody in 1950. This diagram considered the skull or cranium, the tongue bone or hyoid bone, the vertebral column, the shoulder girdle and the associated musclegroups, the muscles of mastication, supra-and infrahyoid muscles and postcervical muscles, all in a side elevational view. The shoulder girdle and the vertebral column were taken to be the fixed base.
The Brody diagram has only been used to explain the connection between the skeletal and muscular components. Orthodontia has only occupied itself with the skeletal parts and the dentition. Anatomy, in practice, has limited itself to the study of the head and the cervical vertebrae upwardly to the atlas, which is the connection between the top of first cervical vertebra and the cranium which permits rotation. Anatomy has considered the further study of the head to belong to the field of dentistry. Dentistry, however, and particularly orthodontia, has only occupied itself with the skeletal parts and the dentition and gives hardly any attention to the functions and the functioning of the muscles.
At least four muscle functions affect the human dentition: posture maintenance of the head, facial expression, mastication, and deglutition. No function is completely independent of the others. In the diagram proposed by Brody it is essentially possible to keep the head in balance during these functions, since changing the head balance will affect all the functions.
According to physiology and anatomy, both sides of our body should be bilaterally symmetrical. Both sides function equally but oppositely, in order to balance the body. In practice, however, dissimilarities between the left and the right side of the body can be observed.
Such imbalances in a bilaterally symmetrical upright position are only possible if compensatory mechanisms are active. A tentative origin for this maintenance of equilibrium can be sought in the activation of muscles which were not originally designed for the function (Thompson and Brody). This maintenance of equilibrium cannot be explained with the Brody diagram, because this equilibrium requires that the shoulder girdle and the vertebral column are no longer considered as the fixed base.
A modification is thus required in order to permit an explanation of an unstable condition resulting from interaction of the various components of the craniomandibular system during bilaterally asymmetrical upright position of the human body. Such a modification would essentially permit the vertebral column to be flexible and twistable, and the scapula (omoplate) and clavicula (collar bones) to be movable with respect to the sternum (breastbone), and the cranium to be movable with respect to the vertebral column.
Consequently, the muscle systems in the area must be differentiated with respect to the vertebral column: the posterior head-equilibrium muscles are represented by the M. Trapezius and the anterior head-equilibrium muscles by the M. Sternocleidomastoideus; the mouthclosing muscles are one group and the mouth-opening muscles are differentiated to the suprahyoid M. Digastricus and the infrahyoid M. Sternohoyoideur and M. Omohyoideus.
This proposed modification gives an essential position to the hyoid bone and to the functions of the supra and infrahyoid muscles. The function and position of the hyoid bone can be influenced by all the structural elements with which it is connected and takes part in nearly all movements of the craniomandibular system such as mouth opening and closing, chewing, and swallowing. In fact, the hyoid bone functions as a second mandible, except for movements during swallowing.
When swallowing, the upper and lower jaw are in occlusion and the hyoid bone and related laryngeal and pharyngeal structures move downwardly. This means, that if the mandible finds itself in a rotated position, the hyoid bone is also rotated and the function of the related structures will be disturbed. In fact, one of the characteristics of the supra- and infrahyoid muscles is supposed to be, that they are not capable of proper functioning when they are stretched or must deal with greater tensions then for which they were originally destined. These muscles and their disposition of nerves induce other muscular structures to take over or to facilitate part of their function, a supposition that is supported by electrographic findings (Davies) pointing out the simultaneous activities of infra-hyoid and posterior cervical muscle system.